Dual tract induction

ABSTRACT

The invention relates to an internal combustion engine wherein two intake tracts 12, 14 balanced for equal air flow, lead to each intake valve and both tracts have outer sections which open on to the same side face of the engine, the inner sections of the tracts having axes which lie in a common plane containing the axis of the valve and the inner sections of the tracts being substantially symmetrical about the valve axis.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an internal combustion engine havingdual tract induction, that is to say two tracts leading to each intakevalve of the engine.

2. Disclosure Information

In racing engines not intended for road vehicles, the intake ports havebeen inclined as near as possible to the valve axis. This makes betteruse of the skirt area of the valve opening and facilitates a ram effectto maximise the intake charge. However, this is not practicable in acommercial engine as the height of the engine is restricted and one isobliged to bend the intake port to permit sensible packaging. Thisbending distorts the flow conditions and introduces an asymmetry causingthe flow to cross the valve stem which introduces turbulence and reducesthe usage of the available skirt area.

GB-971,211 provides two intake tracts leading to each inlet valve fromabove with the aim of restoring symmetry and improving the breathing soas to improve top end performance. Other known designs (see GB-728,487)have two intake tracts, one opening on to the side of the engine in theconventional manner and the other on to the top surface of the engine. Afurther prior art proposal has been to provide tracts from each side ofthe engine leading to the same intake port.

With tracts entering the cylinder head from both sides of the engine,the performance improvement obtained was found disappointing anddevelopment was discontinued. In the case of engines with one or moretracts opening on to the top of the cylinder head, a complex design ofinlet manifold is required to straddle the valve train. Furthermore, theengine needs to be fuel injected as the problems caused by the wetmanifold of a carburetted engine would be difficult to overcome onaccount of the complex manifold design.

In further known proposals, for example GB-1,179,087, U.S. Pat. No.4,550,699, U.S. Pat. Nos. 4,174,686 and GB-1,567,812, two inlet tractshave been used with the intention of creating swirl in the intakecharge. The above patents include proposals for angling the tractstangentially relative to the valve skirt and for introducing differentamounts of air through the two tracts in order to promote swirl. Thepresent invention, on the other hand, is not concerned with promotingswirl and on the contrary takes steps to avoid such swirl.

According to the present invention, there is provided an internalcombustion engine including a cylinder head, intake poppet valves andintake tracts controlled by said valves and leading from a side face ofthe cylinder head to the engine cylinders, there being two tractsleading to each intake valve, characterised in that the tracts aredimensioned and shaped for substantially equal air flow under all engineoperating conditions so as to supply air uniformly to the perimeter ofthe valve skirt without introducing swirl into the intake charge, thetracts being formed of outer sections which extend substantiallyparallel to the base of the head and which open separately on to thesaid side face of the engine and inner sections which lie in a planecontaining the axis of the valve and disposed symmetrically within thelatter plane about the axis of the valve, the inner sections of thetracts remaining symmetrical about the valve axis for a sufficientlength to ensure that the major component of the velocity of the intakecharge is directed along the valve axis.

In the prior art in which the purpose of the separate porting was topromote swirl, it was possible for the ports to open onto the side ofthe engine. However, it was never suggested in the prior art to employside entry porting in arrangements where laminar air flow was to bemaintained as such a port geometry appears to negate the purpose of thedesign, namely to maintain symmetry and equal air flow around the entirevalve skirt perimeter. The present invention is predicated on therealisation that by the use of side entry, an engine with non-swirl twinporting can be packaged to fit under the bonnet of a modern motorvehicle without significantly impairing its operating performance.

Preferably, the second sections are inclined at an angle of less that45° to the valve aXis so that the major component of the velocity of theintake charge should be directed along the valve axis.

The plane containing the axes of the inner sections of the tracts shouldpreferably extend along the engine centre line so that the tracts shouldbe of equal length but if insufficient space is available for two portsection between adjacent intake valves then it is possible for theplanes of the inner tract sections to be inclined to the engine centreline.

It is also convenient that the valve axes should be inclined away fromthe engine centre line as this then calls for a less abrupt transitionfrom the outer section to inner section of each tract.

The invention is concerned with an engine primarily designed for a roadvehicle and differs from all prior art proposals employing dual tractinduction in that the design of the intake porting is intended to takeadvantage not only of the top end performance but of the lean burncapability and of the improved exhaust emissions. These improvementshave not previously been noted because no attempt was made to assess theperformance of the prior art proposals under part load and low speedconditions as the engines were not designed for this purpose. It isbelieved that this is the reason why the prior art proposals havehitherto remained experimental and have never been incorporated in acommercial engine.

It has now been found, very surprisingly, that non-swirl dual tractinduction can achieve significant improvements under low load and partload operation. The surprising nature of this discovery is emphasised bythe fact that an engine with twin tract induction has been made to runwith leaner mixtures than have previously been found possible even usingfour valves per cylinder.

It is generally well known that lean burn is enhanced by introducingturbulence into the charge and with this aim in mind, cylinder headsdesigned for lean burn have generally included means for increasingswirl and charge velocity. It is believed that a mechanism notpreviously appreciated is responsible for the improved lean burncapability of symmetrical twin tract porting. This mechanism involves atoroidal flow being created within the cylinder spilling over from thevalve skirt. The turbulence in the toroid remains confined and bettercontrolled and less kinetic energy is lost through friction with thecylinder walls.

An effect of the toroidal flow, which is believed to be the reason forthe improved performance which has now been discovered, is that the fuelin the charge tends to be better atomised on account of the increasedkinetic energy in the charge.

This explanation of the flow conditions created by symmetrical twintract porting given above is consistent with the fact that in prior artproposals having a vertical tract and one lateral tract, bettercombustion occurred when fuel was injected into the vertical tract thaninto the lateral tract. The reason is that on account of the toroidalnature of the flow, the gases originating in one tract remain alignedwith that tract and consequently the charge was stratified. Bettercombustion of leaner mixtures therefore occurred when a fuelconcentration remained in the vicinity of the spark plug at the end ofthe compression stroke.

It is accordingly a preferred feature of the invention that in a fuelinjected engine, fuel should be introduced into the cylinder throughonly one of the two tracts, namely the tract nearer the spark plug.

In the case of a carburetted engine, metering fuel in dependence uponthe air flow through only one of the two intake tracts is not believedto be sufficiently accurate. However, in such an engine, the presentinvention, by providing access to both tracts from the same side of theengine, permits a one piece branched manifold to be employed to connectboth tracts to a common carburettor. Of course, it is still possible tometer the fuel to only one tract by using a carburettor and in this casethe port design of the engine also simplifies the manifoldconfiguration. For example, one set of tracts may be supplied with aironly through a first manifold and the other set of tracts may besupplied with a fuel/air mixture via a second manifold leading to acarburettor, the two manifolds having ganged throttles.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a side view of a cylinder head,

FIG. 2 is a view along a stepped section line in FIG. 1,

FIG. 3 is a section along the line G--G in FIG. 2,

FIG. 4 is a section along the line Y--Y in FIG. 2,

FIG. 5 is a section along the line Z--Z in FIG. 4, and

FIG. 6 shows a perspective view of the shape of the intake tracts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 shows the shape of the port geometry for each intake valve of anengine. In an engine, the ports are of course hollow passages and FIG. 6may be considered as a cast of the ports.

Each port comprises two separate tracts 12 and 14 leading to the valveskirt area of the intake valve. At their inner sections, the two tractsmerge and form one passage surrounding the valve skirt and coaxial withthe valve stem, the latter being represented schematically in FIG. 6 bythe line 18. Air thus enters the combustion chamber along a passagecoaxial with the valve.

At their outer sections, the tracts 12 and 14 are separate and open ontothe same side of the engine. The transition from the outer sections tothe inner sections is not abrupt to provide for a smooth flow of theinduction air around the two bends. If the intake valves are inclinedoutwards from the engine centre line, then the angle between the innerand outer sections of the tracts is reduced, making for improved flowconditions along the tracts.

The symmetry of the port geometry ensures that equal air quantities aretake in through the two tracts and the air distribution around the valveskirt is uniform. This mode of flow is believed to promote the formationof toroidal vortices and increase the lean burn capability of theengine. As the air spills over the edge of the valve it curls under thevalve head face and sets up rings of turbulence which break away fromthe valve and move like smoke rings into the combustion chamber.

FIGS. 1 to 5 show more detailed views of an engine constructed withintake ports as schematically shown in FIG. 6.

FIG. 1 shows a side view of a cylinder head 10 from the side onto whichthe inlet manifold is fitted. The cylinder head is for a four cylinderengine and has two inlet tracts 12a, 14a to 12d, 14d leading to eachintake valve. The tracts 12, 14 converge onto the intake valve fromopposite sides to provide symmetrical air flow. The various sectionsshown in the drawings are to illustrate the shape of the inlet tracts indetail.

The inner sections of tracts 12, 14 are best shown in the sections ofFIG. 4 and 5. As viewed in FIG. 4, the axis of the valve guide 16 andaxes of the individual tracts are all in the same plane, namely thesection plane of FIG. 5 which is represented by the line Z--Z.

In the section of FIG. 5, it can be seen that the intake tracts aresymmetrical about the the axis of the valve guide. The inner sectionsextend at a small angle to the valve axis, substantially less than 45°in the illustrated embodiment, so that the major component of the airvelocity is parallel to the valve axis resulting in an efficientblending of the two flows.

The different sections of FIG. 2 show the shape of the intake ports atdifferent heights above the valve seat. The view on the left is a trueplan view and the tracts 12a and 14a cannot be seen and are shown indotted lines. The next section to the right passes through the entireouter sections of the tracts 12b and 14b and the passages leading fromthe inlet manifold to the opposite sides of the valve are clearly shown.

It will be noted that in the case of the embodiment illustrated, theouter section of the tract 14b is slightly longer than the outer sectionof the tract 12b. The reason for this is that the valve axis does not inthis case lie in a transverse plane but is inclined at a compound angleto the engine axis. The plane containing the axis of the valve istherefore also slightly canted as can be seen from the angle of thesection line Y-Y in FIG. 2 which passes through the valve axis. Thisslight asymmetry, caused by the adoption of an existing valve train,does not present serious cause for concern and can be compensated by theshape of the intake manifold. The important dimension is the totallength of the intake tracts, rather than the length of any one sectionand the lengths can be balanced if the lengths of the branches of theinlet manifold leading to the tracts 12 are slightly longer than thelengths leading to the tracts 14.

The section of FIG. 3 shows the generally rectangular shape of theindividual tracts 12 and 14 but this is not essential. The shape ischosen to optimise the through flow cross section within the constraintplaced by the spacing of the cylinders and the need to package the otheressential engine components.

Beneath the tracts 12 and -4 in FIG. 3, there are shown cooling passages20 of the water jacket for cooling the intake air. FIG. 1 also showspassages for water cooling of the intake manifold.

In designing air intake ports, three different approaches havepreviously been adopted and each has its own strengths. The crosssection may, according to these three approaches, converge gradually,remain constant or diverge gradually. It is believed that any one ofthese three approaches may be adopted in the port design in the presentinvention, but the relative merits of these approaches at differentvalve openings have yet to be assessed It is believed to be importantthat the port should be designed such that at a given valve height, thecontinuity of the intake manifold convergence or divergence ismaintained up to the discharge area into the combustion chamber.

It will further be noted that the port illustrated in FIG. 6 includes aventuri throttle adjacent the valve seat. The shape of this portion ofthe port is known to be of importance to the engine performance and indesigning an engine, the shape of this portion and the shape of thevalve head should be carefully selected to maximise air flow at theoptimum valve opening. If necessary, the valve head should be formed onthe port side to conform to the required port area and shape andhollowed out on the chamber side to achieve the required shape withoutincreasing the mass of the valve.

The fuelling of the engine may be either by carburettor or by aninjection system. In the latter case, the injectors are preferablysituated on whichever of the tracts is nearer the spark plug. It isfeasible, to use a carburettor to fuel only one set of tracts but thisis not preferred as metering errors may occur, especially during idleconditions.

I claim:
 1. An internal combustion engine having a centerline parallelto the crankshaft of said engine, wherein two intake tracts, balancedfor equal air flow, lead to each intake valve, with each such valvehaving an axis, and both tracts having outer sections which open on tothe same side face of the engine, the inner sections of the tractshaving axes which lie in a common plane containing the axis of the valveand the inner sections of the tracts being substantially symmetricalabout the valve axis.
 2. An engine as claimed in claim 1, wherein theplane containing the axes of the inner sections of the tracts (12,14)extends parallel to the engine centre line so that the tracts should beof equal length.
 3. An engine as claimed in claim 1 wherein the valveaxes are inclined away form the engine centre line.